Superabrasives, such as CBN, generally cost about one hundred to one thousand times as much per unit volume as other conventional abrasives but nevertheless are cost effective in grinding certain materials such as tool steels.
CBN is used predominantly in grit sizes from about 100-400 for precision grinding processes. In general in the prior art the total grinding performance of a tool containing CBN has been found to correlate closely with the volume fraction of CBN in the tool, although a certain amount of porosity in a grinding wheel containing CBN is usually desirable. Conventional abrasives have been used together with other superabrasives in abrasive tools in the prior art, but the conventional abrasive in such products has functioned primarily as a kind of filler and has contributed little if anything as an abrasive to the total cutting tool life when the tool was used on hard work pieces such as tool steels. For example, "A" Makhlouf et al. U.S. Pat. No. 4,652,277 of Mar. 24, 1987 refers to the use of silicon carbide and alumina to help make the coefficients of thermal expansion of the rim and core regions as close to each other as possible in a grinding wheel containing CBN in the rim region only, but there is no suggestion that these conventional abrasive materials contribute directly as abrasives to the cutting performance of the wheel.